Digital photography has overtaken traditional film based photography as the routine means by which images are taken and stored. Initially confined to single purpose camera devices, over time digital cameras incorporated into cellular phones grew increasingly popular, and at the present time, the majority of digital images captured by individuals for non-commercial purposes are captured using a digital camera incorporated within a cellular phone. Accordingly, as the desire for cellular phones capable of acting as digital cameras grew, the desire for the digital images captured by those cellular phones to be of higher quality grew.
For years, the image sensors used within the digital camera systems of cellular phones increased in resolution (i.e. pixel count), with top of the line cellular phones incorporating image sensors having more than 16 million pixels (MP), and in one case, over 40 MP.
This increase in resolution was accomplished in part by shrinking the size of individual pixels. This in turn, in prior art designs, resulted in each pixel having a reduced charge storage capacity, which means that each pixel captures less light. Since the maximum signal to nose ratio is a function of the square root of the charge storage capacity, these smaller pixels ultimately result in a worse signal to noise ratio.
Consequently, the trend to greater numbers of smaller pixels began to reverse, and the current trend is toward larger pixels with greater charge storage capacity. Since image sensors for cellular phones are desired to be small and compact, the challenge is therefore to design sensors with larger pixels of sufficient count (resolution), yet keep the sensor size as compact as possible.
Due to this challenge, rolling blade operated type pixels are commonly employed instead of global shutter operated pixels, due to the traditionally smaller area occupied by rolling blade shutter pixels. However, as will be explained, rolling blade shutter pixels have drawbacks compared to global shutter pixels.
In a rolling blade shutter, an array of pixels are processed line by line, with one being integrated and another being read out for each movement of the shutter. The shutter moves over the array so that the pixels are exposed for the same amount of time, but not at the same moment in time. Therefore, it is evident that a rolling blade shutter may not work well when taking images of fast moving objects.
With a global shutter, the pixels of the array are simultaneously released from reset and start to integrate simultaneously. As a result, the drawbacks of a rolling blade shutter are not present. After a specific period, the pixels are then read out simultaneously into a temporary storage, which may be located inside the pixel. This temporary storage is then scanned out row by row where the signal is amplified or converted into a digital value.
Since the pixels integrate simultaneously, each pixel has at least one dedicated storage capacitor, and in more advanced cases, each pixel may have two dedicated storage capacitors. An example ten transistor global shutter pixel with two output bit lines VX0A and VX0B is shown in FIG. 1. The pixel 1 includes a number of transistors M1 to M11, the functions of which will be set out in more detail below. It should be noted that the capacitors C1 and C2 are shown as being MOS transistors configured to act as capacitors, but may also be metal-insulator-metal capacitors. The pixel 1 also includes a photodiode Pd, a floating diffusion capacitor Cfd, and input and output lines (VDD, VRT, TG, VBIAS, READ1, SAMPLE1, SAMPLE2 and RESET).
M4 is a transfer gate transistor to transfer charge during pixel reset from VRT (flowing through M2) to the photodiode Pd, and during pixel readout to transfer charge from the photodiode Pfd to the floating diffusion capacitor Cfd. M2 is used to reset the floating diffusion capacitor Cfd and if TG is high, the photodiode Pd is also reset if RST is simultaneously asserted.
M1 is a source-follower where the voltage on the source of M1 follows the voltage on the gate of M1, which is set by the voltage across the floating diffusion capacitor Cfd. M5 is an active load for M1 to help ensure it operates correctly. To save power, it is possible to pull VBIAS low when the pixels are not being read out so that M1 is not used.
Transistor M6 is used as a switch and when enabled, allows the voltage at the source of M1 to be stored on the capacitor C1. As stated, C1 can be a metal-insulator-metal capacitor (as shown in FIG. 2), but as the capacitor is storing a voltage and not a charge and is followed by a source-follower transistor (M7), it does not need to be linear and hence the gate of a MOS transistor can be used as the capacitor (as shown in FIG. 1A). M7 is a source follower for the voltage on the storage capacitor C1.
M9 provides the same functionality as M6, but for a second storage site at capacitor C2. M10 provides the same functionality as M7, but for the second storage site. C2 can be a metal-insulator-metal capacitor or metal-insulator-metal capacitor and provides the same functionality as C1, but for the second storage site.
M3 is a read transistor and is enabled when the signal from the corresponding row is required. It is disabled when another row in the sensor is being accessed. M11 provides the same functionality as M3, but for the second storage site.
The use of the two storage capacitors C1 and C2 enables two separate images to be handled separately, as they are written to and read from independently. The two storage capacitor C1 and C2 are each written to respectively at first and second times, which times correspond to the successive frames captured. These storage capacitors C1 and C2 are traditional two plate capacitors.
As shown, the pixel 1 is formed on a single wafer 2. The result of this is that the pixel size is large, leading to a reduced number of pixels for a given area. Therefore, while an image sensor made using the design of this pixel 1 has advantages in terms of improved signal to noise ratio, as well as the capabilities afforded by the independently writeable and independently readable capacitors, such an image sensor would have drawbacks in terms of total resolution. Since higher resolutions permit a higher degree of digital zooming, as well as display of larger images without visible quality degradation, it would be desirable to overcome these drawbacks, and to have the advantages of this pixel design without the drawbacks. Accordingly, further development has been needed.